1. Field of the Invention
The present invention relates to a pH measuring electrode for use in measuring the pH of a solution and a method of manufacturing the same. More particularly, it relates to a pH measuring electrode in which a metal oxide is used as a sensitive film (sensor film) thereof and a method of manufacturing the same.
2. Description of the Related Art
As is well known, to measure the pH of a solution, a pH measuring electrode having a sensor film which is sensitive to a hydrogen ion (H.sup.+) in the solution is used. A typical example of such an electrode is a glass electrode in which the glass is employed as a sensor membrane. Recently, ISFET electrodes have also come to be used in a limited scope of fields. Furthermore, for a special solution, such as a high temperature solution or the like, or for a special application, a pH measuring electrode in which a metal such as platinum or the like, or a metal oxide such as titanium oxide or the like is employed as a sensitive film thereof, is used.
When pH of a solution is actually measured, said pH measuring electrode is used as a working electrode and is immersed in a solution to be measured together with a reference electrode such as a calomel electrode, silver-silver chloride electrode or the like, and a pH value of the solution to be measured is determined from the electric potential difference between both the electrodes.
Since glass electrodes are quite stable and are capable of making highly accurate measurements, they are greatly used to measure the pH of various kinds of solutions. However, glass electrodes are liable to be easily broken, and therefore they are troublesome to handle. Further, because they have poor resistance to chemicals, there is a limitation in the number of solutions pH of which can be measured by the glass electrodes. In addition, there is a problem in that, since an electric resistance value of the glass membrane is high, glass electrodes are difficult to miniaturize from the viewpoint of current manufacturing technology, which is a serious disadvantage as there is an increased demand for the miniaturization of pH meters and other pH measuring instruments.
For this reason, various types of electrodes for measuring pH which are capable of eliminating these drawbacks have been investigated, and the pH measuring electrodes in which the above-mentioned metals or metal oxides are used as sensitive films thereof, or ISFET electrode, have been provided.
However, the above-described conventional pH measuring electrodes in which metals or metal oxides are employed as sensitive films thereof have problems in that the range of pH the electrodes can measure is quite limited depending upon the kind of sensitive film employed therein, and that for a short period of time, stable measurements of pH cannot be made. For example, if there is a coexistent substance such as an oxidizing agent or a reducing agent in a solution to be measured, a great drift in electric potential measured by the electrode is caused and an accurate pH measurement could not be made. Also, ISFET electrodes have such problems that their drifts are great, that they are affected by light, that special amplifying circuits are required, etc.
The inventors have confirmed, after various kinds of experiments have been carried out, that to use iridium oxide as a sensitive film of a pH measuring electrode is effective. Well-known thin-film production technology such as sputtering, ion-beam deposition, ion plating or CVD, was used to form an iridium oxide film on an insulation substrate. However, in case that an iridium oxide film was formed on an insulation substrate, there is a limitation in the shape of the substrate. Also, since it is required to cover the lead-out portion of a lead wire secured to the iridium oxide film with a bonding agent, its workability is poor. In addition, it is difficult to make such electrode small at will.